The present invention relates to an architecture for a satellite transmitter/receiver subsystem in a satellite based telecommunications system. More specifically, the present invention relates to an adaptive transmit and receive architecture which reallocates communications resources in real time based on the amount of bandwidth being used by the system to support communications channels.
The Federal Communications Commission (FCC) has recently made available for commercial use certain bandwidth spectra in the 26 to 40 GHz frequency range. In response to the availability of this new high frequency range, several communications companies have bid for and obtained portions of this spectrum from the FCC to use in trying to develop systems of communicating through one or more satellites around the globe at very high data rates. Today, fast modems operate at a data transmission rate of 56 kilobits per second. The newly proposed satellite communications systems operating at frequencies between 26 to 40 GHz have data transmission rates on the order of 10 megabits per second or higher, approximately 200 times greater than the data transmission rate of today's fast modems.
These new satellite based telecommunications systems have been proposed to provide voice and/or date communications links between user terminals (mobile and fixed) and earth stations. The earth stations, in turn, connect the user terminals with remote originating/destination callers through public land mobile networks (PLMN), public switching telephone networks, other earth stations, other satellites, and the like. Each user terminal communicates with an assigned earth station along corresponding forward and return links which are supported by a satellite which has the user terminal and earth station in its field of view.
Each satellite includes at least one antenna which defines its earth coverage region or footprint. The satellite antenna(s) divide the coverage region into multiple beam spots. Each beam spot is assigned at least one frequency subband along which communications signals travel in the forward and return directions between user terminals and earth stations. Each subband may support communications from a plurality of user terminals. The user terminals are assigned unique transmission channels or "circuits" within an associated subband. A channel or "circuit" represents a unique path along which the corresponding user terminal transmits and receives radio frequency (RF) signals containing discrete frames or packets of communications data and/or command information. A channel or circuit may be defined in a variety of ways, depending upon the system's coding technique, such as time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA) or any combination thereof.
FIG. 1 generally illustrates a block diagram of a conventional receiver architecture of a telecommunications satellite. The receiver architecture includes an uplink antenna 1 with N beams 3 corresponding to N respective geographic supercells. Each beam 3 is divided into a predetermined number of subbands 5. The subbands 5 have been numbered from SB# 1 to SB# 553. In the example of FIG. 1, each beam 3 includes seven subbands 5. For purposes of illustration only, it is assumed that each beam 3 is divided into seven subbands 5 each of which corresponds to a unique carrier frequency.
Each user terminal is assigned to one variable output broadcast matrix 7 that corresponds to one beam. There are N variable output broadcast matrices 7 contained in the receiver architecture of FIG. 1 (e.g., in the uplink structure) corresponding to the N beams into which the overall footprint of the satellite is divided. Each beam 3 of the satellite's uplink antenna is divided into seven subbands 5 and therefore in the examples of FIG. 1, a 7.times.7 output broadcast matrix 7 is used. That is, the variable output broadcast matrix 7 can have up to seven inputs and seven outputs. Thus each variable output broadcast matrix 7 can handle communications signals from user terminals in up to seven subbands. The N variable output broadcast matrices 7 are connected to RF downconverters 9 (e.g., LO1-LO7). Each downconverter 9 processes seven subbands associated with the beam corresponding to that 7.times.7 broadcast matrix 7. Within one given broadcast matrix 7, each user terminal is allocated an amount of frequency bandwidth based either on a peak (or full) bandwidth capacity, or on a "butter-spread" capacity.
Where the frequency bandwidth is allocated using peak capacity, one channel for one user terminal will occupy all seven subbands within the beam 3. A single channel may be defined in terms of a single chip code and time slot for a system using both CDMA and TDMA techniques. Thus, a single channel may use the seven subbands 5 being fed into a broadcast matrix 7 to form a single communications signal from one user terminal. Once this signal is fed into the broadcast matrix 7, the matrix allocates a portion of the signal to each of the seven variable local oscillators (LO1-L07) or downconverters 9 at the output of the matrix 7. The LO1-L07 downconverters 9 mix the input signal to a corresponding predetermined carrier frequency. The mixed output signals of the downconverters 9 are then filtered by a bandpass filter 11, summed by a seven-way summer 15, and then processed by the analog to digital (A-D) converter 17 dedicated to the particular beam through which the communications signal was received.
When the bandwidth is allocated in an "butter-spreading" manner, each channel, and thus each user terminal, occupies a single subband 5 within the beam 3. After each of the seven signals are fed into the broadcast matrix 7, the broadcast matrix 7 passes the seven signals to their corresponding downconverters 9 at the output of the matrix 7. The downconverters 9 mix corresponding input signals to a predetermined carrier frequency. The outputs of the downconverters 9 are then individually filtered by a respective bandpass filter 11, summed by a seven-way summer 15, and then processed by the analog to digital (A-D) converter 17 which is dedicated to the particular beam by which the user terminals' signals are received.
In order for satellite based telecommunications systems to operate, each satellite in the system must contain sufficient processing hardware to accommodate the maximum amount of information being transferred at any instant in time. Heretofore, conventional systems required that each satellite contain a separate processing unit or hardware subsystem for each beam of the satellite. For example, a satellite with 100 beams must contain 100 separate, individual processing units. As satellites move across regions of the earth that differ in population density, the demands of any given satellite greatly fluctuate.
Various factors affect the congestion of users in each beam spot at any given moment in time. Accordingly, a system requiring a separate processing unit for each individual beam is extremely inefficient when several beam spots are supporting little or no communications channels. For example, as the time of day changes, the number of active users in a given beam spot or footprint rapidly changes. At 8:00 AM in New York City, the number of users in that geographical area is high, while at the same time, it is 5:00 AM in Los Angeles and thus the user activity and, more generally, the user population in that area is likely to be quite low. Similarly, at 5:00 PM in Los Angeles, the number of active users is likely to be much higher in L.A. than in New York City, where it is 8:00 PM. Prior art systems have addressed the foregoing issue in one of two ways. One approach has been to determine before the fact the most efficient use and allocation of processing hardware. A second approach has been to assume maximum demand 100% of the time and provide sufficient hardware in every satellite to handle the demand. In the former situation, satellites are designed to provide much lower processing capabilities for beamspots which cover areas with few user terminals. In other words, satellites and/or beam spots that are expected to cover less populated areas (or areas that are projected to have few users) are built with limited transmitter/receiver hardware resources. However, a given satellite will be used for many years. Thus, the potential exists for change in the geographic population. Satellites of the former design cannot account for increases in demand or in population. Thus if, for example, the future population is underestimated for a given area, the resulting lack of communication capacity would be very expensive in the long run since a new satellite would be required.
On the other hand, in the latter situation, each satellite contains sufficient hardware to provide the maximum amount of processing capability for every one of that satellite's beam spots. A beam spots compability would then only be limited by the available bandwidth and channels capable of being supported by the available bandwidth. Thus, a beam spot that covers Chicago will have the same amount of channel or circuit capacity as a beam spot that covers central Africa. This is a very inefficient use of resources. Further, the extra hardware required to support the maximum number of channels would require a heavier launch vehicle, create unnecessary complexity, and require a greater amount of power consumption. The power consumption would be larger because a processor for any given beam must operate at full capacity regardless of whether one or one-thousand user terminals are communicating within that beam spot coverage area. This results in an extremely inefficient use not only of processing resources, but of power consumption as well.
As explained above, the foregoing telecommunications satellite transceiver structure has met with limited success, as the transmitter/receiver hardware is unduly complex and/or inefficient. A need remains for an improved satellite transmitter/receiver architecture (hereafter transceiver).